Common Terms

What do the various Magnetic Terms mean?

Remanence (Br) is the flux density of a magnetic material in closed circuit, which remains after the removal of the magnetising field. Remanence is measured in Gauss, Tesla or mT. (1 Tesla = 10,000 Gauss)

Flux Density (B) is a measure of magnetic field strength of the magnet in an 'open circuit' condition. The actual flux density measured on the pole face of a magnet will depend on the material, the grade, the relationship of it's pole area to it's magnetic length and any additional pole pieces that create a further magnetic circuit. Flux density is measured in Gauss, Tesla or mT.

Coercive Force (Hc) is the strength of the demagnetising field needed to reduce the flux density of the magnet to zero. B=0 (bHc)Coercive force is measured in Oersted or kA/m.

Maximum Energy Product BH (max) indicates the peak energy that a magnet can deliver when operating at a working point on the demagnetisation curve. Maximum Energy Product is measured in Mega-Gauss-Oersteds or kJm³.

A Demagnetisation curve is the second quadrant of a magnet hysteresis loop for a magnetic material and defines the main magnetic properties of a permanent magnet.

A 'B-H Curve' plots on a graph, the cycling of a magnet in a closed circuit as it is saturated, demagnetized, saturated in the opposite direction, and then demagnetized again under the influence of an external magnetic field.

The second quadrant of the B-H curve, commonly referred to as the "Demagnetization Curve", describes the conditions under which permanent magnets are used in practice. Every type and size of magnet will have a unique, operating point based on material, geometry and circuit and this will determine the open circuit (B) value. If the circuit changes shape, or the magnet changes shape then the operating point will move about the demagnetization curve and the (B) value will change accordingly.

The three most important characteristics of the B-H curve are the points at which it intersects the B and H axes (at Br- the residual induction - and Hc - the coercive force - respectively), and the point at which the product of B and H are at a maximum (BH (max)- the maximum energy product). Br represents the maximum flux the magnet is able to produce under closed circuit conditions. In actual useful operation permanent magnets can only approach this point. Hc represents the point at which the magnet becomes demagnetized under the influence of an externally applied magnetic field. BH (max) represents the point at which the product of B and H, and the energy density of the magnetic field into the air gap surrounding the magnet, is at a maximum. The higher this product, the smaller need be the volume of the magnet. Designs should also account for the variation of the B-H curve with temperature.

When plotting a B-H curve, the value of B is obtained by measuring the total flux in the magnet (ø)and then dividing this by the magnet pole area (A) to obtain the flux density (B=ø/A). The total flux is composed of the flux produced in the magnet by the magnetizing field (H), and the intrinsic ability of the magnet material to produce more flux due to the orientation of the domains. The flux density of the magnet is therefore composed of two components, one equal to the applied H, and the other created by the intrinsic ability of ferromagnetic materials to produce flux. The intrinsic flux density is given the symbol Bi where total flux B = H + Bi, or, Bi = B - H. In normal operating conditions, no external magnetizing field is present, and the magnet operates in the second quadrant, where H has a negative value. Although strictly negative, H is usually referred to as a positive number, and therefore, in normal practice, Bi = B + H. It is possible to plot an intrinsic as well as a normal B-H curve. The point at which the intrinsic curve crosses the H axis is the intrinsic coercive force, and is given the symbol Hci.

High Hci values are an indicator of inherent stability of the magnet material. The normal curve can be derived from the intrinsic curve and vice versa. In practice, if a magnet is operated in a static manner with no external fields present, the normal curve is sufficient for design purposes. When external fields are present, the normal and intrinsic curves are used to determine the changes in the intrinsic properties of the material.

The Working Point is the point on the demagnetisation curve where the value of B & H correspond to the actual working conditions of the magnet. The load line for a particular shape of magnet and its associated circuitry, drawn from the origin, cuts the demagnetisation curve at the working point. For static applications there is one working point, there may be several for a dynamic system.

The L / D ratio of magnet (length to diameter ratio) is used to establish the load line angle and thus the working point for magnets used in an open circuit condition.

Isoptropic Magnets have magnetic domains that are randomly orientated and therefore they can be magnetised equally in any direction. Only the magnetic domains that lie in the particular direction of magnetisation can be used.

Anisotropic Magnets have their magnetic domains locked in one particular direction during manufacture. They can ONLY be magnetised along this axis. Anisotropic magnets are much stronger than isotropic magnets because all the magnetic domains are used in the same direction.

© Copyright e-Magnets UK 2008